The resolution and scene contrast of an imaging radar system is determined by the system's ability to constrain within a single resolution pixel the primary response of a point target return. Ideally, the return energy from a point scatterer would be contained within one resolution cell with no energy in any other cell, but this would require an infinite observation time window. As a practical compromise, radar systems are designed to maximize the point target return energy within one resolution cell while minimizing the remaining return energy in all other resolution cells. This requires minimization of waveform and pulse distortions occurring within a system's signal paths, as those distortions can cause the return energy to be distributed across multiple resolution cells.
All items in the RF signal path of a radar system may potentially impart phase, amplitude, and time delay distortions on the signal, and the imparted distortion undesirably results in the signal being spread over a broader range of frequencies and time. Several approaches have previously been pursued to reduce this waveform distortion. One approach is simply to specify and manufacture all RF hardware components to such tight tolerances that their total contribution to distortion remains within allowable levels. This is expensive, and does not assure the hardware will meet all requirements under all operating conditions.
Another approach, if the characteristics of the imparted distortion are known in advance of normal operation, is to apply a complementary inverse predistortion factor to the transmission and/or reception of an operational signal in order to compensate for the imparted distortion. The availability of programmable waveform synthesizers in exciters and of digital signal processors in, or associated with, receivers allows such predistortion to be applied, but in order to determine what sort of compensatory factor to apply, the distortion must be characterized and measured in advance. One approach to such distortion measurement involves use of a calibration loopback signal that is generated in the exciter and routed by special distribution circuitry through portions of the antenna RF distribution network and back into the receiver for observation of total generated distortion. This approach can characterize distortion contributed by certain portions of the system's signal path, but cannot characterize distortion contributed by the active circuitry in the antenna's transmit/receive chain, and so tight tolerances must continue to be maintained on all those antenna components. This approach also introduces a new source of distortion, the special distribution circuitry itself.
Another approach involves use of external test and reference signals generated by and/or analyzed by separate system test equipment. This approach may be able to characterize distortion contributed by the antenna transmit/receive chain, but it requires availability of the external system test equipment whenever and wherever calibration is desired, which is impractical for applications such as military and aerospace. This approach also introduces a new source of distortion, the external test equipment itself.